Shade area under lines doesn't work with negative values

InfluxDB has an option to shade area under lines. However, shading should always be between the line and the x-axis. InfluxDB shades the area between the bottom of the chart and the line. This causes very strange behavior for negative values.

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Create a data source that has negative and positive values
2. Create a chart and select "shade area under lines"
3. Notice that the area is below the lines is always shaded, even if the value is negative. With negatives values, the area between the x-axis and line should shaded.

__Expected behavior:__
Area between x-axis and line shaded.

__Actual behavior:__
Area between x-axis and bottom of chart is shaded (similar to Chronograf).

__Environment info:__

* System info: Linux 5.4.0-77-generic x86_64
* InfluxDB version: InfluxDB 2.0.8
* Other relevant environment details: n/a

InfluxDB 2.0.8:

Chronograf:

Start by reproducing the chart with mixed negative and positive values and the “shade area under lines” option. Trace the chart-rendering path responsible for the shaded region; done means the fill is between each line and the x-axis, including for negative values, without changing positive-value behavior.

Written by the indexing model from the issue text.